System and method for monitoring changes in state of matter with terahertz radiation

ABSTRACT

A system and method for using terahertz radiation to detect and monitor a substance undergoing a change in phase from a liquid phase to a solid phase or vice-versa is disclosed. By employing terahertz radiation in either the pulsed mode or in the continuous-wave (CW) mode, the system can non-invasively monitor these changes. The system uses the principle that matter in a liquid state will absorb and attenuate terahertz radiation to a larger degree than matter in a semisolid or solid state. Most terahertz radiation absorption occurs due to the rotational motions of molecules, i.e. either whole molecules or groups of atoms rotating about molecular bonds.

FIELD OF INVENTION

[0001] The present invention relates to a terahertz (THz) radiationdetection and analysis system. More specifically, the present inventionrelates to a terahertz radiation detection and analysis system andmethod used to detect phase changes in matter.

BACKGROUND

[0002] Presently there is no commercially available device or method tonon-invasively or non-destructively monitor phase changes in substances,such as adhesives or glue as they cure or dry. Furthermore, there are noknown devices or methods that can monitor adhesive curing when theadhesive is sandwiched between two adjoined parts, such as two pieces ofpaper, two sheets of plastic, layers in laminated wood or ceramics, orglass.

[0003] Accordingly, there is a need in the art to monitor the processingof such a curing procedure to insure the integrity of an adhesive bondand to monitor the quality of various products.

SUMMARY OF THE INVENTION

[0004] Terahertz radiation (electromagnetic radiation in the range of 50GHz to 10 THz) both pulsed and continuous-wave, can be used for thispurpose. Terahertz radiation will be absorbed and attenuated differentlywhen it passes through matter in a liquid state, semisolid state, orsolid state. These attenuation differences can be detected and monitoredto detect the state of a sample in the process of changing phase such asan adhesive undergoing a curing process.

[0005] The invention comprises an apparatus and method, using terahertzradiation, that allows the detection and monitoring of many differentmaterials as they change from the liquid phase to the solid phase orvice-versa. By employing terahertz radiation in either the pulsed modeor in the continuous-wave (CW) mode, a system can non-invasively monitorthese changes. The terahertz system of the present invention uses theprinciple that matter in a liquid state will absorb and attenuateterahertz radiation to a larger degree than matter in a semisolid orsolid state. Most terahertz radiation absorption occurs due to therotational motions of molecules, i.e. either whole molecules or groupsof atoms rotating about molecular bonds. THz radiation is more highlyabsorbed by more polar rotating moieties. Rotational motion occursreadily when a material is in the liquid state, however, as a materialhardens or freezes, this kind of motion is substantially restricted,thus making the material more transparent to terahertz radiation. Mostliquid adhesives are highly polar, providing a strong contrast betweenthe absorption of the freely rotating liquid adhesive molecules and thecured adhesive whose molecules cannot rotate.

[0006] The same physical properties which allow terahertz radiation tobe used to monitor the curing of glue, i.e. the transition from a liquidstate to a solid state, also allow terahertz radiation to be used tomonitor other liquid-solid or solid-liquid phase changes such as waterto ice and vice-versa. This is true for ice or for frozen objects suchas frozen food containing water. Furthermore, terahertz radiation may beused to monitor the amount of water in moisture critical commercialproducts such as powdered drinks and baby food.

[0007] The advantage of the terahertz monitoring system is itsversatility and ease of use in an industrial environment. The terahertzsystem of the present invention is ruggedly packaged and can be used inan industrial environment for the processing of the aforementionedcommon commercial products such as epoxy, glue, ice cubes, baby food,and frozen food, but is not limited to such. The terahertz system may beused to monitor the curing of adhesive used to couple materials such ascardboard, laminated sheets of wood or plastic, caulking, siliconesealant, and other types of adhesives. Furthermore, the terahertz systemmay also be used to monitor the drying of paints, such as on a car body.

[0008] By taking advantage of the varying absorption properties ofterahertz radiation, with respect to phase changes in matter, aterahertz device can be employed to monitor these types of phasechanges.

[0009] Further objects, features and advantages of the invention willbecome apparent from a consideration of the following description andthe appended claims when taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a diagrammatic view of a time domain terahertz system ofthe present invention;

[0011]FIG. 2 is a diagrammatic view of a continuous wave terahertzsystem of the present invention;

[0012]FIGS. 3 and 3a are graphs illustrating the absorption of terahertzradiation for melting ice;

[0013]FIG. 4 is a graph illustrating the absorption of terahertzradiation for a frozen cucumber that has been melted and then re-frozen;and

[0014]FIGS. 5, 5a, and 5 b are graphs illustrating the absorption ofterahertz radiation for curing epoxy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015]FIG. 1 is a diagrammatic view of a terahertz electromagneticradiation emission and detection system shown generally as 10. Anoptical source 12 comprising a Ti:sapphire laser producing sub-100femtosecond pulses at 800 nm is coupled to a precompensator 14. Althougha Ti:sapphire laser is the preferred optical source 12, other shortpulse sources may be used such as: a modelocked Er-doped fiber laserfrequency doubled to produce pulses at 750-800 nm; a colliding-pulsemodelocked (CPM) laser; an amplified Ti:sapphire laser consisting of aseed pulse that is amplified to higher energies; a frequency-doubled,modelocked Nd based glass laser; a modelocked laser based on any of thechromium doped hosts: LiCaF, LiSrAlF, or LiSrGaAlF; or any laser sourceproducing femtosecond output pulses at megahertz repetition rates, butis not limited to such. Although the preferred embodiment uses a lasersource operating at around 800 nm, a source such as an Er doped fiberlaser, operating at 1550 nm may be used if the appropriatesemi-conductor material is also used in the transmitter and receiver.

[0016] In operation, the output pulse from the optical source 12 issplit by a fiber splitter 17 to single mode optical fibers 16 and 18. Inorder to achieve a transform-limited pulse at the output of the singlemode optical fibers 16 and 18, a precompensator 14 is used to adddispersion of a sign opposite to the dispersion acquired in the fibers16 and 18. Dispersion is the name given to the property of groupvelocity variation with wavelength. This will tend to spread, stretch,and/or distort an optical pulse shape, making it indistinct. Thesimplest form of dispersion comes from the propagation of light throughbulk material. The source of this dispersion is the non-linearfrequency-dependent index of refraction. The precompensator 14 may becomprised of gratings, holographic gratings, prisms, grisms, Bragg-fibergratings, Gires-Tournier interferometer, or any other combinationthereof that results in a negative group velocity dispersion system. Theoptical fibers 16 and 18 can comprise numerous commercially availablesingle mode fibers.

[0017] As the optical pulse exits the optical fiber 16 it will travelthrough a fiber optic delivery apparatus 22 to strike a terahertztransmitter 24, which will emit a single-cycle or half-cycle ofelectromagnetic radiation in the terahertz frequency range. Thepreferred embodiment of the terahertz transmitter 24 employs aphotoconductive element, generating electron-hole pairs and an impulseelectrical current. The photoconductive element may be a pn-junctiondiode, pin photodiode, metal-semiconductor-metal photodiode,point-contact photodiode, heterojunction photodiode, or a simplesemiconductor, which can be fabricated with any semiconductor elementcomprised of low temperature grown GaAs, semi-insulating-GaAs, Silicon(crystalline or ion-implanted) on Sapphire, InAs, InP, InGaAs, or anyother photoactive element but is not limited to such. Thephotoconductive element used to generate a terahertz pulse can also beof the kind outlined in U.S. Pat. No. 5,420,595 entitled “MicrowaveRadiation Source” which issued to Zhang et al. on May 30, 1995, and isincorporated by reference herein.

[0018] A current pulse will be generated by the optical pulse strikingthe photoconductive element of the terahertz transmitter 24. Thevariation in current will generate electromagnetic radiation in theterahertz frequency range. The temporal shape of the electromagneticradiation is determined both by the shortness of the input optical pulseand the metal antenna structure that is coupled to the photoconductiveelement. In the preferred embodiment the antenna is in a dipoleconfiguration. The antenna configuration for this preferred embodimentis outlined in U.S. Pat. No. 5,729,017, “Terahertz Generator andDetector”, which issued to Brenner et al. on May 17, 1998, and isincorporated by reference herein. The radiation in the preferred modewill be from 50 gigahertz to 100 terahertz, but any electromagneticfrequency above or below this preferred range is possible.

[0019] The terahertz radiation is transmitted through optical elements26 which condition the terahertz radiation. The conditioned terahertzradiation then passes through a sample 28 and a second optical element30 to a terahertz receiver module 32. As discussed previously, phasechanges in the sample 28 can be characterized by a frequency-dependentabsorption, dispersion, and reflection of terahertz transients insignals which pass through the sample 28. By monitoring the total energyof the received terahertz radiation passing through the sample 28,material phase changes may be monitored. The terahertz radiationreceiver 32 in FIG. 1 is configured to detect electromagnetic radiationin the terahertz range, after the terahertz radiation has passed throughthe sample 28. The terahertz radiation receiver 32 can be placed at anyposition surrounding a sample 28, so as to detect absorbed, reflected,refracted or scattered radiation. The terahertz radiation receiver 32will then generate an electrical signal proportional to the power orenergy of the received terahertz radiation which is subsequentlyamplified by amplifier 34 and interpreted, scaled, and/or digitized by adata acquisition system 36.

[0020] The terahertz receiver 32 is synchronized to the terahertztransmitter 24 by optical pulses traveling through optical fiber 18 andfiberoptic delay 20 controlled by a trigger device (not shown). Thefiber optic delay 20 will control the gating of the received terahertzsignal.

[0021] The system described herein represents the preferred embodimentused to perform the demonstration. However, a pulsed, time-domain systemcould be based on electro-optic generators and other detectors could beused as well. Other embodiments would consist of all electronic methodswith Gunn diodes or non-linear transmission lines as transmitters andbalometers as detectors.

[0022]FIG. 2 is a diagrammatic figure of an alternate terahertztransmitting and receiving system utilizing a continuous wave system.Two semiconductor diode lasers 42 and 44 are optically coupled toproduce a continuous wave signal at optical coupling point 46. Thecontinuous wave signal is generated by the constructive and destructiveinterference of the diode laser 42 and 44 outputs. The laser 42 and 44may be modulated to generate any desired frequency. Similar to the firstembodiment of the present invention shown in FIG. 1, the continuous waveis applied to a terahertz transmitter 24′ that generates terahertzradiation. The terahertz radiation is transmitted through opticalelements 26′ which condition the terahertz radiation. The conditionedterahertz radiation then passes through a sample 28′, a second opticalelement 30′, and a terahertz receiver module 32′. The signal from theterahertz receiver 32′ is analyzed similar to the first embodiment. Thecontinuous terahertz radiation generated by the system 40 will enablemeasurements that are wavelength sensitive, such as the monitoring of agaseous material with a sharp absorption line. The continuous wavesystem configuration, of this preferred embodiment, is outlined infurther detail in U.S. Pat. No. 5,663,639, “Apparatus and Method forOptical Heterodyne Conversion”, which issued to Brown, et al. on Sep. 2,1997, and is incorporated herein by reference.

[0023]FIGS. 3 and 3a are illustrations of pulsed waveforms transmittedthrough a cube of ice as the cube of ice slowly melts over time. Thegraphs 50 and 52 illustrate the amplified voltage signal of theterahertz receiver 32 versus time. As can be seen, the transmittedpower, as measured by the voltage signal, from the terahertz radiationpassing through the ice slowly decreases as the water in the beam pathstarts to increase.

[0024]FIG. 4 shows a series of pulsed waveforms taken as a frozencucumber slice starts to melt, and then again after it has beenre-frozen. The graph 54 illustrates the amplified voltage signal of theterahertz receiver 32 versus time. Similar to FIGS. 3 and 3a the watercontent of the frozen cucumber determines the attenuation of theterahertz radiation.

[0025]FIGS. 5, 5a, and 5 b show a series of waveforms taken as standard,two-part, five-minute epoxy cures. The graphs 56 and 58 illustrate theamplified voltage signal of the terahertz receiver versus time and thegraph 60 illustrates the integrated terahertz power as detected versustime. Since the epoxy is curing, i.e. going from a liquid state to asolid state, this signal voltage is increasing in strength over time.

[0026] Graphs 56 and 58 were taken during an experiment thatcontinuously monitored the total energy transmitted through a 1 cm pathlength of epoxy as it cured. Both FIGS. 5 and 5a show the same energytrend for the epoxy, during the first 5 minutes of curing. The graphs 56and 58 show about the same transparency to terahertz radiation with aslight decrease occurring at 4-5 minutes after mixing, this dip isfollowed by a steady rise in transmitted energy over the next 10minutes.

[0027] With specific reference to FIG. 5b, an example of an applicationof terahertz radiation to monitor liquid to solid phase changes inglues, epoxies, water, and other similar material is illustrated. Graph60 shows the transmitted terahertz power through a cuvette of standard2-part, 5 minute epoxy. As the epoxy starts to polymerize, the cuvetteof epoxy becomes more transparent to the terahertz radiation, asindicated by the increase in transmitted terhertz power over time.Initially, as the two parts are mixing, the cuvette of epoxy appears tobecome temporarily more opaque (transmitted terahertz power decreases),just before the solidification begins.

[0028] Since, the phase transition from liquid to solid and the freezingof rotational vibrations in most substances are usually detectable inthe terahertz regime, there is good reason to believe that a similarresult would be found for other glues.

[0029] The graphs 50, 52, 54, 56, 58, and 60 were taken using a pulsedor time-domain terahertz system. In the preferred embodiment of thepresent invention, a continuous wave (CW) Terahertz system like thatshown in FIG. 2 will be used in industrial applications. This systemwould constantly monitor the total transmitted power at a specificterahertz frequency, rather than intermittently monitor terahertzradiation over a broad wavelength range.

[0030] It is to be understood that the invention is not limited to theexact construction illustrated and described above, but that variouschanges and modifications may be made without departing from the spiritand scope of the invention as defined in the following claims.

1. A system for determining whether a sample under inspection isundergoing a change in state, the system comprising: a laser lightsource for generating laser light; an optically-driven terahertztransmitter which converts the laser light into terahertzelectromagnetic radiation, wherein the terahertz electromagneticradiation is transmitted through the sample; an optically-driventerahertz receiver positioned opposite the terahertz transmitter forreceiving the terahertz electromagnetic radiation transmitted throughthe sample; and an analyzer for comparing the terahertz electromagneticradiation transmitted by the optically-driven terahertz transmitter tothe terahertz electromagnetic radiation received by optically-driventerahertz receiver to determine whether the sample is undergoing achange in state.
 2. The system of claim 1 wherein the analyzer furthercompares a transmitted power of the terahertz electromagnetic radiationtransmitted by the optically-driven terahertz transmitter to a receivedpower of the terahertz electromagnetic radiation received by theoptically-driven terahertz receiver to determine whether the sample isundergoing a change in state.
 3. The system of claim 1 wherein theanalyzer further compares a transmitted peak voltage amplitude of theterahertz electromagnetic radiation transmitted by the optically-driventerahertz transmitter to a received peak voltage amplitude of theterahertz electromagnetic radiation received by the optically-driventerahertz receiver determine whether the sample is undergoing a changein state.
 4. The system of claim 1, wherein the laser light source is alaser capable of producing an optical pulse having a duration ofapproximately 100 femtoseconds.
 5. The system of claim 4, wherein afiber delivery system is used to deliver the optical pulse from thelaser light source to the optically-driven terahertz transmitter andreceiver.
 6. The system of claim 5, further comprising a precompensatorfor adding dispersion to cancel dispersion acquired in the fiberdelivery system.
 7. The system of claim 1, wherein the laser lightsource further comprises a plurality of single-frequency light sourcesfor generating a plurality of single frequency-light signals, whereinthe plurality of single frequency-light signals are added coherently atthe optically-driven terahertz transmitter and receiver.
 8. The systemof claim 7, further comprising a fiber combiner for spatiallyoverlapping the plurality of single frequency-light signals.
 9. Thesystem of claim 1, wherein the terahertz transmitter and receiverfurther comprise a photoconductive element for converting the laserlight to the terahertz electromagnetic radiation.
 10. The system ofclaim 9, wherein the photoconductive element is a low-temperature-grownGaAs semiconductor.
 11. The system of claim 1, wherein the terahertztransmitter and receiver further comprise an electro-optic element forconverting the laser light to the terahertz electromagnetic radiation.12. The system of claim 11, wherein the electro-optic element iscomprised of ZnTe.
 13. The system of claim 1, wherein the terahertztransmitter and receiver further comprise an antenna for improvingcoupling efficiency of the terahertz electromagnetic radiation out ofthe transmitter and into the receiver
 14. The system of claim 1, furthercomprising an optical delay device for introducing a delay in thetransmission of the laser light from the laser light source to theoptically-driven terahertz receiver.
 15. The system of claim 1, furthercomprising a terahertz optical element in alignment with the terahertztransmitter for focusing the terahertz electromagnetic radiation ontothe sample.
 16. The system of claim 1, further comprising a terahertzoptical element in alignment with the terahertz receiver forconcentrating the terahertz electromagnetic radiation onto the terahertzreceiver.
 17. A method for determining whether a sample under inspectionis undergoing a change in state, the method comprising: generating acoherent lightwave using a laser light source; converting the coherentlightwave into terahertz electromagnetic radiation using anoptically-driven terahertz transmitter, wherein the optically-driventerahertz transmitter transmits the terahertz electromagnetic radiationthrough the sample; receiving the terahertz electromagnetic radiationtransmitted through the sample using an optically-driven terahertzreceiver positioned opposite the optically-driven terahertz transmitter;and comparing the transmitted terahertz electromagnetic radiation to thereceived terahertz electromagnetic radiation to determine whether thesample is undergoing a change in state.
 18. The method of claim 17,wherein comparing the transmitted terahertz electromagnetic radiation tothe received terahertz electromagnetic radiation further comprisescomparing a transmitted power of the terahertz electromagnetic radiationtransmitted by the optically-driven terahertz transmitter to a receivedpower of the terahertz electromagnetic radiation received by theoptically-driven terahertz receiver for determining whether the sampleis undergoing a change in state.
 19. The method of claim 17, whereincomparing the transmitted terahertz electromagnetic radiation to thereceived terahertz electromagnetic radiation further comprises comparinga transmitted peak voltage amplitude of the terahertz electromagneticradiation transmitted by the optically-driven terahertz transmitter to areceived peak voltage amplitude of the terahertz electromagneticradiation received by the optically-driven terahertz receiver fordetermining whether the sample is undergoing a change in state.
 20. Themethod of claim 17, further comprising producing an optical pulse havinga duration of approximately 100 femtoseconds.
 21. The method of claim20, further comprising delivering the optical pulse to theoptically-driven terahertz transmitter and receiver using a fiber opticdelivery system.
 22. The method of claim 21, further comprising addingdispersion using a precompensator to cancel dispersion acquired in theoptical delivery system.
 23. The method of claim 17, further comprisingimproving coupling efficiency of the terahertz electromagnetic radiationout of the optically-driven terahertz transmitter and into theoptically-driven terahertz receiver using an antenna, wherein each thetransmitter and receiver include the antenna.
 24. The method of claim17, further comprising focusing the terahertz electromagnetic radiationonto the sample using a terahertz optical element in alignment with theoptically-driven terahertz transmitter.
 25. The method of claim 17,further comprising focusing the terahertz electromagnetic radiationtransmitted through the sample onto the optically-driven terahertzreceiver using a terahertz optical element in alignment with theterahertz receiver.
 26. The method of claim 17, wherein generating acoherent lightwave using a laser light source further comprisesgenerating a plurality of single frequency-light signals using aplurality of single frequency-light sources, wherein the plurality ofsingle frequency-light signals are added coherently at theoptically-driven terahertz transmitter and receiver.
 27. A system fordetermining whether a sample under inspection is undergoing a change instate, the system comprising: a pulsed laser light source for generatinga pulse of laser light; an optically-driven terahertz transmitter whichconverts the laser light into a pulse of terahertz electromagneticradiation, wherein the terahertz electromagnetic radiation istransmitted through the sample; an optically-driven terahertz receiverpositioned opposite the terahertz transmitter for receiving theterahertz electromagnetic radiation transmitted through the sample; afiber delivery system for transmitting the laser light from the laserlight source to the optically-driven terahertz transmitter and receiver;an optical delay device for introducing a delay in the transmission ofthe laser light from the laser light source to the optically-driventerahertz receiver; and an analyzer for comparing the terahertzelectromagnetic radiation transmitted by the optically-driven terahertztransmitter to the terahertz electromagnetic radiation received by theoptically-driven terahertz receiver to determine whether the sample isundergoing a change in state.
 28. The system of claim 27, wherein thelaser light source is a laser capable of producing an optical pulsehaving a duration of approximately 100 femtoseconds.
 29. The system ofclaim 27, further comprising a precompensator for adding dispersion tocancel dispersion acquired in the fiber delivery system.
 30. The systemof claim 27 wherein the analyzer further compares a transmitted power ofthe terahertz electromagnetic radiation transmitted by theoptically-driven terahertz transmitter to a received power of theterahertz electromagnetic radiation received by the optically-driventerahertz receiver to determine whether the sample is undergoing achange in state.
 31. The system of claim 27 wherein the analyzer furthercompares a transmitted peak voltage amplitude of the terahertzelectromagnetic radiation transmitted by the optically-driven terahertztransmitter to a received peak voltage amplitude of the terahertzelectromagnetic radiation received by the optically-driven terahertzreceiver determine whether the sample is undergoing a change in state.